In the emerging area of parallel and parallel coarse wavelength division multiplexing (CWDM) optical interconnects, the trend is to increase the number of channels per module while maintaining channel speed at a reasonable level as determined by the available device and integrated circuit technologies. For example, in one system, it may be desired to run a 48 channel module at a speed of up to 10 Gb/s. In such a module, channels are typically arranged into four groups of twelve with each group operating at a different wavelength. The testing and evaluation of a system having many high speed channels presents a problem. Extension of typical conventional approaches where each channel is handled separately requires a test configuration that uses a large number of high-speed cables, connectors, terminations, a large evaluation board and a considerable number of man-hours for testing and evaluation. Hence, it is desirable to transition from the typical conventional approach to a more integrated approach that, for example, uses compact on-board components allowing the selection of a subset of channels at the time of measurement while properly terminating the remaining channels. This allows a significant reduction in the number of high-speed cables and components needed, reducing the complexity of the test configuration. Central to such an integrated test approach is a switch that allows easy selection of one or more channels from a multiplicity of high speed channels.
A switch that allows easy channel selection is a high-speed channel selector switch (HCSS) as disclosed in U.S. Pat. No. 6,933,628 hereby incorporated by reference.
The HCSS typically consists of two units-stationary unit 100 and rotary 200 unit as shown in FIGS. 1 and 2, respectively. High speed channel pads 120 are distributed around circle 125 in a circular pattern and a high speed channel is electrically coupled to each of high speed channel pads 120. Rotary unit 200 has a pad frame that matches to stationary unit 100 resulting in a one to one mapping between outer pads 220 on rotary unit 200 and high speed channel pads 120 on stationary unit 100 when rotary unit 200 is properly mated to stationary unit 100. Placing conductive balls or bumps on stationary unit 100 allows high speed connections to be made. On rotary unit 200, all outer pads 220 except one are terminated to proper loads or impedances by short segments of transmission lines 250. The remaining outer pad 220 is electrically coupled to pad 240 at the center by transmission line 225. This allows selection of one channel while properly terminating all others to termination impedances 275. Note that portions 260 of rotary unit 200 are kept at ground. By rotating rotary unit 200 by the proper amount, any high speed channel is selectable.
However, the HCSS switch allows only one channel to be selected at a time and may be a problem for some test applications. For example, in applications that involve differential signaling and two lines, the use of the HCSS switch allows monitoring of only a single line at a time. Hence, monitoring both lines simultaneously is not possible with the use of one HCSS switch. Additionally, the mechanical tolerances required for the HCSS switch are not easily implemented. Because pads 140 and 240 are at the center of stationary unit 100 and rotary unit 200, respectively, alignment features such as an alignment pin cannot be located at the center. The mechanical housing typically has several parts, a rotary part, a stationary part and a clamping part. The inability to use the center for alignment of these parts typically degrades the stack-up tolerances. The mechanical housing functions to hold and lock the rotary unit with respect to the stationary unit and when unlocked the mechanical housing allows the rotation of the rotary unit in fixed increments. To allow large stack-up tolerances requires the use of larger pads 140 and 240 that degrade the high speed performance of the pads because of the increasing pad capacitance associated with increased pad size.